1. Field of the Invention
This invention pertains generally to broadband mirrors, and more particularly to high reflectivity gratings.
2. Description of Related Art
Semiconductor light emitting diodes and lasers are used in a wide range of applications, such as telecommunication, display, solid-state lighting, sensing, surveillance and imaging. For many of these applications it is desirable to have devices having light emission normal to the surface of the wafer. The surface emitting topology facilitates array fabrication, integration with other devices and wafer-scale testing during and after processing. One form of these light emitting devices requires integration of mirrors with high reflectivity.
Broadband mirrors (Δλ/λ>15%) with very high reflectivity (R>99%) are essential for numerous applications, including telecommunications, surveillance, sensors and imaging, ranging from 0.7 μm to 12 μm wavelength regimes. For example, in optical integrated circuits, electro-optic modulators play an important role in switching and signal encoding. Ideally, electro-optic modulators have low insertion loss and wide bandwidth. Mirrors are key components and the performance of many modulators would be substantially improved if they incorporated a low insertion loss, broad bandwidth, mirror. In the case of surface-emitting semiconductor light-emitting diodes and lasers, the broadband mirrors are required in the construction of optical cavity resonators to achieve large quality (Q) factors.
Among the candidates for mirrors are metal mirrors and dielectric mirrors. Metal mirrors have comparatively large reflection bandwidths but lower reflectivities (R), as they are limited by absorption loss. As a result, they are not suitable for fabricating transmission-type optical devices such as etalon filters.
Dielectric mirrors on the other hand have a lower loss than metal mirrors and therefore can achieve a higher reflectivity. However, the available deposition methods are often not precise enough to readily provide these high reflectivities. It should be appreciated that dielectric mirrors are composed of multi-layer dielectric materials with different dielectric indices. Distributed Bragg Reflectors (DBR) consist of multiple periods of alternating high and low refractive index layers. The tuning range for a tunable filter made with DBR mirrors is determined by the DBR mirror bandwidth and the maximum allowable mechanical movement, whichever is smaller. These mirrors have low absorption loss, but the modulation depth, bandwidth and band location depend on the refractive index contrast of the constituent materials as well as on the control over the layer thickness.
In order to minimize interface disorder and strain in the multilayer structures, typical combinations of materials often have small refractive index differences, thus resulting in very limited bandwidths (Δλ/λ≈3-9%). As a result of this narrow bandwidth, the tuning range of electro-optic modulators, such as etalon type devices, has been severely limited.
For tunable etalon type devices, such as micro-electro-mechanical (MEM) vertical cavity surface emitting lasers (VCSEL), filters and detectors, the tuning range is often limited by semiconductor based distributed Bragg reflectors (DBRs) to Δλ/λ≈3-9%. Conventional designs have not provided a mirror with broadband reflection, low loss and compatibility with optoelectronic processing. Semiconductor-based DBRs have been widely used for vertical cavity surface emitting lasers (VCSEL), detectors, and filters because of their higher thermal and electrical conductivities. A typical VCSEL requires an optical resonant cavity having two DBRs, wherein one DBR is positioned on each side of a cavity layer. In the center of the cavity layer resides an active region. In one implementation the active region comprises at least one layer of quantum wells or quantum dots. Current is injected into the active region through a current guiding structure such as provided by either an oxide aperture or proton-implanted surroundings. The laser emission wavelength of the structure is determined by the Fabry-Perot resonance wavelength of the cavity and DBRs, as well as the active region gain bandwidth. The use of semiconductor DBRs within devices limits the emission wavelength, wherein a mirror with high reflectivity and broad bandwidth is desired.
One of the major difficulties in the current status of VCSEL fabrication, especially for long wavelength components around 1.55 μm regimes, concerns the realization of high quality reflective p-type VCSEL mirror. This fabrication difficulty is primarily a result of the limited choice of materials available for the material growth process. In conventional semiconductor-based DBR (e.g., AlxGa1-xAs) devices, the refractive index contrast is low, such as somewhere between the high and low index material. Consequently, an excess number of DBR pairs is required to achieve >99% reflectivity, thus increasing the difficulty in achieving high quality material growth. This has been a major bottleneck for 1.3-1.55 μm VCSEL fabrication and remains a problem for blue-green and 2-3 μm wavelength regimes. The shortcomings of this prior approach are even more pronounced with regard to the fabrication of wavelength-tunable VCSELs, in which the requirements on mirror bandwidth and reflectivity become even more stringent.
Accordingly, there is a need for vertical cavity surface emitting lasers which can be readily fabricated across a range of wavelengths as well as for wavelength-tunable devices. The present invention satisfies that need, as well as others, and generally overcomes the limitations of the art.